The present invention pertains to an aqueous slurry for making a powder of hard material (e.g., hard carbide and metallic binder). More specifically, the present invention pertains to an aqueous slurry for making a powder of hard material such as, for example, a cemented (cobalt) tungsten carbide, and the powder of hard material, as well as an article made there from. The powder of hard material and the article made there from each possess properties that essentially meet properties of a powder of hard material formed from a solvent (i.e. Heptane)-based hard material slurry, as well as the articles made there from, respectively.
Cemented carbides (e.g., cemented (cobalt) tungsten carbide (WC—Co)) are typically used in demanding applications that require high strength, wear resistance, and high temperature deformation resistance. Strict and consistent quality control of the cemented (cobalt) tungsten carbide powders used to produce the sintered article is important to achieve products with the desired properties. Solvent-based (e.g., heptane) processing is a common and established technique to produce cemented (cobalt) tungsten carbide powders, but this technique requires rigorous environmental, health, and safety standards because of the risks associated with the flammability and disposal of the solvents.
As a result of these issues connected with the use of solvents, there has been an interest in using an aqueous slurry as a milling medium for the milling hard material components such as hard carbides and metallic binders. As set forth hereinafter, there are certain advantages inherent in using an aqueous milling slurry in contrast to using a solvent such as, for example, Heptane, in the milling slurry. The paper by Nebelung and Thiele entitled “Technology of Aqueous Hard Metal Processing” presented at the Powder Metallurgical Symposium at Hagen on Nov. 26-27, 2009 lists a number of patent documents that pertain to aqueous hard metal processing. Other publications that pertain to aqueous hard material processing include Laarz et al., “Dispersing WC—Co powders in aqueous media with polyethylenimine”, International Journal of Refractory Metals & Hard Materials, 18 (2000), pp. 281-286, and a Doctoral Thesis from the Royal Institute of Technology, Stockholm, Sweden (2004) entitled “Aqueous Processing of WC—Co Powders” by Karin M. Andersson that includes the following articles: Andersson and Bergström, “Oxidation and Dissolution of Tungsten Carbide Powder to Water”, International Journal of Refractory Metals & Hard Materials, 18, pp. 121-129 (2000); Andersson and Bergström, “DI, VO Interactions of Tungsten Oxide and Cobalt Oxide Surfaces Measured with Colloidal Probe Technique”, Journal of Colloid and Interface Science, 246, pp. 309-315 (2002); Andersson and Bergström, “Effect of the Cobalt Ion and Polyethyleneimine Adsorption on the Surface Forces between Tungsten Oxide and Cobalt Oxide in Aqueous Media”, Journal of the American Ceramic Society, 85, [10], pp. 2404-2408 (2002); Laarz, Jonsson and Andersson, “The Effect of Dispersant Addition and Binder Content on the Properties of Spray-Dried WC—Co Granules”, manuscript in preparation; Andersson and Bergström, “Density Measurements of Single Granules using the Atomic Force Microscope”, submitted to the Journal of the American Ceramic Society; and Andersson and Bergström, “Friction and Adhesion of Single Spray-Dried Granules containing a Hygroscopic Polymeric Binder, submitted to Powder Technology. 
One exemplary patent document is U.S. Pat. No. 3,846,126 to Foley et al. (filed on Jan. 15, 1973), which discloses the use of water as the solvent and polyvinyl alcohol as the binder. The Foley et al. patent discloses the use of other binders at Col. 3, lines 53-62 wherein these binders include camphor, methyl alcohol, paradichlorobenzene, chloroacetic acid, naphthalene, benzoic acid, phthalic anhydride, glycerine, Acrowax C, ethylene oxide polymers sold as Carbowax, and synthetic gums such as acrylamide and metal stearates. In Foley et al. water is the preferred vehicle for milling unless an increase in oxygen content is a concern in which case use of a solvent is sought. When the oxygen content is critical, use of a solvent is the suggested component. Other patent documents are identified and discussed hereinafter.
U.S. Pat. No. 4,070,184 to Scheithauer et al. has as a focus the use of a water soluble relatively long chain polyglycol additive. The Scheithauer et al. patent sets out the basic differences between itself and the then prior art at Col. 3, lines 37-45:                The present invention is an improvement over the most modern practice used today for preparing carbide grade powders. It involves three basic and radical departures from the common practice.        1. The use of water as a milling fluid as opposed to flammable organics.        2. The use of an open-cycle spray-drying system as opposed to closed system.        3. The use of water soluble, long-chain polyvinyl alcohol as a mixing aid instead of paraffin wax. The basic advantages of the process of this invention are cost, safety, flexibility of operation, and product improvement.The Scheithauer et al. patent describes the spray drying process at Col. 5, lines 36-46:        Next the slurry is transferred to the spray dryer feed tank. It is heated, to about 50° C., and agitated while the Carbowax 6000 addition is made. This addition is generally 1-3%. For this grade it is preferably 2%. At this point the spray drying process begins. A suitable spray drier is a Proctor-Schwartz spray tower with two-fluid top-nozzle atomization. Some of the important drying parameters are air pressure of 20 psi, an inlet drying temperature of 200°-230° C. and an outlet temperature of 100°-130° C.Another patent that discloses the use of water in the milling of hard materials include U.S. Pat. No. 4,397,889 to Benjamin et al. (filed Apr. 5, 1982). U.S. Pat. No. 4,478,888 to Benjamin et al. discloses spray drying at [Col. 4, lines 33-54]:        Spray drying may be carried out using commercially available spray drying equipment. The inlet and outlet air temperatures should be maintained below about 370° C. and 190° C., respectively, to prevent substantial oxidation or decarburization of the slurry constituents. The spray drying is carried out under conditions to produce an agglomerated powder mixture consisting essentially of agglomerated particles of metal carbide, metal binder and wax. Typically the size range of the agglomerated particles is from about 20 to about 150 microns. During spray drying, the slurry is generally heated to about 50° C. and agitated. A suitable spray dryer is a Protco-Schwartz spray dryer with two-fluid-top nozzle atomization. When water is the milling fluid, typical drying parameters may be an air pressure of 20 psi, drying temperature of 200°-230° C. and an outlet temperature of 100°-130° C. When organic solvents are used as the milling fluid, the spray drying is preferably performed in absent air. Spray drying temperatures are dependent on the volatility of the solvent. The spray dried agglomerates may be classified by screening to obtain a desired fraction.        
U.S. Pat. No. 4,886,638 to Penkunas et al. mentions the use of water in the slurry used to make a hard material. U.S. Pat. No. 4,902,471 to Penkunas et al. discloses a slurry with an organic additive (see Col. 3, lines 16-17):                The organic compound can be an ester of a fatty acid. Some preferred esters are beeswax and carnuba wax. Some preferred fatty acids are lauric acid, myristic acid, palmitic acid, stearic acid, and combinations thereof.U.S. Pat. No. 5,007,957 to Penkunas et al. (and U.S. Pat. No. 5,045,277 to Penkunas et al.) mentions using water and an esterfied wax (see Col. 2, lines 24-29):        The first step in forming the grade powder is to combine the binder metal powder with a solid esterified wax to form a first mixture. The preferred waxes are pure beeswax, carnauba wax, candellila wax and combinations of these, although other esterified waxes can be used.        
U.S. Pat. No. 5,922,978 to Carroll discloses the use of deoxygenated water as a liquid component of the slurry with a preferred composition is at Col. 4, lines 10-16:                In a most preferred embodiment, the method comprises mixing, in essentially deoxygenated water, WC powder, Co and the organic binder described above. The WC preferably has a submicron particle size. The Co preferably has a submicron particle size. The organic binder is preferably a paraffin wax. More preferably the organic binder is a paraffin wax provided as an emulsion in water.The Carroll '978 patent also mentions triethanolamine as a corrosion inhibitor (Col. 4, lines 17-28):        Depending on the first powder and additional component, a corrosion inhibitor, such as those known in the art (e.g., corrosion inhibitors useful in the boiler, machining and heat exchanger art), may be used. If added, the corrosion inhibitor should be one that does not, for example, hinder the densification of a part pressed from the pressable powder. Preferably the corrosion inhibitor does not contain an alkali metal, alkaline earth metal, halogen, sulfur or phosphorous. Examples of corrosion inhibitors include those described in U.S. Pat. Nos. 3,425,954; 3,985,503; 4,202,796; 5,316,573; 4,184,991; 3,895,170 and 4,315,889. Preferred corrosion inhibitors include benzotriazole and triethanolamine.U.S. Pat. No. 6,245,288 to Carroll also uses deoxygenated water to avoid oxygen pickup, and also discloses spray drying is the preferred drying method. See Col. 4, lines 60-64.        
European Patent Application 1 440 956 A1 to Kruse (Seco Tools AB) has as a focus on using polyethylimine-based polyelectrpolyte in water and/or alcohol. In general, the Kruse patent application discloses a method of making cemented carbide bodies based on tungsten carbide and with a binder phase based on Co or combinations of Co, Ni, and Fe, or Ni and Fe by powder metallurgical methods including wet milling in alcohol or water or a mixture thereof, of powder and pressing agent to form a slurry, drying the slurry to form a granulate by spray drying, pressing the granulate to form bodies of desired shape and dimension and finally sintering.
U.S. Pat. No. 6,852,274 to Knünz et al., as well as its European counterpart European Patent No. 1 373 586 B1, disclose a spray drying process that comprises steps of: forming a sprayable slurry of hard material and metallic binder and water and spraying the slurry without the aid of a water-soluble long chain polyglycol. The parameters for the sprayable slurry are: a sprayable slurry with water as a liquid phase and having a solid particle concentration within a range of 65-85% by weight. The parameters of the spray drying are: a gas inlet temperature of substantially 160° to 220° C. and a gas exit temperature in a range of substantially 85° to 130° C., and a ratio of water added with the slurry, in liters per hour, to a tower volume of the spray tower, in m3, lies between 0.5 and 1.8 and such that a maximum of 0.17 kg of slurry is atomized per m3 of incoming drying gas. U.S. Pat. No. 6,733,562 to Knünz et al., which has a European counterpart as European Patent No. 1 373 585 B2, discloses a process that calls for the formation of a slurry of a hard material and metal binder and water wherein after formation of the slurry, the process requires the formation of an emulsion of a non-water soluble pressing aid and an emulsifier and water, which is then mixed with the slurry.
U.S. Pat. No. 6,656,976 to Bergstrom et al., which has European counterpart European Patent No. 1 153 652 B1, pertains to a well-dispersed slurry of mixture of WC-based and Co-based particles and water wherein one feature is the further component of a dispersant comprising 0.1-10 wt % of a polyethylenimine-based polyelectrolyte. U.S. Pat. No. 7,387,658 to Grearson et al., which has European counterpart European Patent 1 739 197 B1, discloses using water as the liquid medium along with about 1 to about 3 wt-% of a pressing agent of equal to or less than about 90 wt-% PEG [polyethylene glycol] and equal to or greater than about 10 wt-% of long chain C≧20 fatty acids, their esters and salts. U.S. Pat. No. 6,878,182 to Kruse discloses a method that includes wet milling in water the powders and pressing agent wherein the slurry is formulated to contain 0.02-0.06 wt % of a polyethylenimine-based polyelectrolyte to the cemented carbide slurry containing WC and Co.
U.S. Pat. No. 7,539,637 B2 to Jutterström et al. pertains to a method of making an agglomerated cemented carbide powder that includes wet milling, preferably in a milling liquid comprising water and/or alcohol or a mixture of water and acetone, a powder mixture containing hard constituent powder(s) and a metallic binder and pressing agents and spray drying the slurry. Before milling, from about 0.05 to about 0.50 wt-% of a complex forming and/or pH-decreasing/increasing additive such as triethanolamine, hydroxides or acids, for example, and a thickener in an amount of thickener from about 0.01 to about 0.10 wt-% is added.
U.S. Pat. No. 7,666,349 B2 to Laarz et al. (European Patent No. 1 806 175 B1 is a counterpart) concerns a method of gel-casting a cemented carbide body. One step in the method is forming an aqueous slurry of WC—Co. The text at Col. 4, lines 22-27 presents a general description:                More specifically, the method according to the present invention comprises the forming of a slurry including WC—Co cemented carbide powder and dispersant in an aqueous medium, to which is added a mono-functional monomer, a cross-linker and a suitable initiator for the system.More details about the method are set forth at Col. 4, lines 28-61.        
U.S. Pat. No. 7,285,241 B2 to Puide concerns injection molding or extruding a hard material component. One step in the process is wet milling the raw materials in water, alcohol or a combination thereof, preferably 80 wt % ethanol, and 20 wt % water, together with the ethylene oxide polymer. More details are set forth at Col. 2, line 55 through Col. 3, line 2.
U.S. Pat. No. 7,303,722 B2 to Bruhn et al. discloses a method to make a hard metal article using powder injection molding or an extrusion method. The method includes a step of wet milling in water or alcohol or a combination of water and alcohol, and the drying the slurry. The text at Col. 3, lines 3-7 provides a basic description of the wet milling:                1. Wet milling of the raw materials in water, or alcohol, or a combination thereof, preferably 80 wt % ethanol and 20 wt % water, together with 0.4-0.8 wt %, preferably 0.5-0.7 wt % stearic acid as a granulating agent for the subsequent spray drying.        
U.S. Pat. No. 6,336,951 B1 to Qvick et al. discloses a method of making submicron tungsten carbide tool inserts. The method includes wet milling using ethylalcohol and water as a milling liquid. See Col. 2, lines 23-35.
PCT Publication WO98/00256 to Sandvik AB concerns a method of spray drying powder mixtures that includes spray drying cemented carbide slurries consisting of cemented carbide powder containing hard constituents in an alcoholwater solution. The text at page 3, line 31 through page 4, line 28 describes the slurry.
European Patent No. 0 963 454 B1 to Sandvik Akiebolag concerns a method of making cemented carbide by powder injection molding. According to this patent, the use of a surfactant in the milling step of the cemented carbide provides for a reduction in the level of the porosity in the sintered part. The surfactant can be a single fatty acid like hexadecanoic acid, tetradecanoic acid, 9,10 Octadecanoic acid, 9,12 Octadienoic acid or 9,12,5 Octadecatrienoic acid mixed with the powder in ethanol, acetone, benzene. Furthermore the surfactant can be some kind of organometallic compound, Zn-stearate, or corresponding alcohol to a fatty acid such as 1-hexadecanol. It can also be an amine such as octadecylamine. All these surfactants can be milled in ethanol. Paragraphs [0011] through [0015] provide more details about the method.
U.S. Pat. No. 7,531,022 to Quirmbach et al., which has European counterpart European Patent No. 1 666 616 B1, discloses a method of using a liquid in the preparation of powder mixtures on the basis of hard metals. The method comprises the steps of: (a) providing a milling liquid comprised of water and an inhibitor wherein the inhibitor being a polyvinyllactam or a mixture of a polyvinyllactam and a wax emulsion; (b) providing a powdered metal comprised of at least one hard metal; (c) combining said liquid with said powdered metal in an attritor to form a moist powder mixture; and (d) atomizing said moist powder mixture in a spray drying installation to produce a powder mixture.
United States Published Patent No. US2007/0259970 A1 to Boden et al., of which European Patent No. 1 742 726 B1 is a family member, pertains to a method for dispersing and passivating particulate powders in water and aqueous media. Water-soluble polyvinylamines and/or the initial products thereof, such as, e.g., polyvinyl formamides, are used for dispersing particulate powders in water and/or aqueous media and furthermore for passivating non-oxidic particulate powders in water.
Although the uses are with solvents, the surfactant Ethomeen has been used in slurries that include tungsten carbide and cobalt. In this regard, U.S. Pat. No. 3,888,662 to Boeckeler and U.S. Reissue Patent RE 34180 to Nemeth et al. discloses the use of Ethomeen.
EthomeenPatentCitation(details)SolventU.S. Pat.EXAMPLE 1, Col.Ethomeen1,1,1,trichloroethaneNo. 3,888,6628, lines 57-60S/14U.S. Pat.EXAMPLE III, Col.EthomeenperchloroethyleneNo. 3,888,66213, lies 10-26S/15USRE34180EXAMPLE NO. 1,EthomeenperchloroethyleneCol. 7, lines 25-36S-15USRE34180EXAMPLE NO. 4,EthomeenperchloroethyleneCol. 9, line 53S-15USRE34180EXAMPLE NO. 5,EthomeenperchloroethyleneCol. 10, line 63S-15USRE34180EXAMPLE NO. 6,EthomeenperchloroethyleneCol. 11, line 23S-15USRE34180EXAMPLE NO. 7,EthomeenSoltrol 130 (a solvent)Col. 12, line 19S-15USRE34180EXAMPLE NO. 12,EthomeenperchloroethyleneCol. 16, line 18S-15USRE34180EXAMPLE NO. 13,EthomeenperchloroethyleneCol. 17, line 20S-15USRE34180EXAMPLE NO. 14,EthomeenperchloroethyleneCol. 17, line 57S-15
While the above documents show that others have used water as a milling medium for the milling of hard materials, there remain drawbacks to the use of water as a milling medium for hard materials such as, for example, cemented (cobalt) tungsten carbide. Some of these drawbacks include insolubility of the binders, surfactants and other additives in water, which results in powders (a powder if hard material) with inhomogeneous and inconsistent properties. This insolubility has generated the necessity to mill the slurry for a longer time in order to achieve the specified powder properties. Milling for a longer time increases the production costs and essentially reduces the production capacity of a ball mill, as well as increases the amount of iron contamination during the milling using a steel-lined ball mill. A higher content of iron contamination can increase the potential for metallurgical defects in the sintered article. Another drawback with aqueous milling is the potential for the powder to oxidize.
Other properties that would be advantageous to possess include providing a process that results in powder of a hard material that exhibits an increase in the granule size of the powder batch, achieves narrow granule size distributions, is easier to discharge the powder slurry from the ball mill, and possesses a reduction in the pressing pressure necessary for satisfactory compaction of the powder into the green body which is thought to minimize die wear as well as cracking in the sintered article.
Therefore, it would be highly desirable to provide an aqueous slurry for making a powder of hard material that does not require a longer milling time in order to achieve the specified powder properties. A reduction in the milling time decreases the production costs and essentially increases the production capacity of a ball mill, as well as decreases the amount of iron contamination during the milling. A lower content of iron contamination can decrease the metallurgical defects (e.g., porosity) in the sintered article. It would also be highly desirable to provide an aqueous slurry for making a powder of hard material that results in powder of a hard material that exhibits an increase in the granule size of the powder batch, achieves narrow granule size distributions, is easier to discharge the powder slurry from the ball mill, and possesses a reduction in the pressing pressure necessary for satisfactory compaction of the powder into the green body which is thought to minimize die wear as well as cracking in the sintered article.